Aluminum oxide coatings deposited on cemented carbides generally with either an intermediate coating of one or more of TiC, Ti(C,N), TiN compounds or a gamma-phase enriched substrate are well-known. Products made according to, for example, Hale (U.S. Reissue Pat. No. 32,110) and Lindstrom et al. (U.S. Reissue Pat. No. 31,520) describe such products which have enjoyed considerable commercial success in their use as inserts for metal cutting.
In my earlier U.S. Pat. Nos. 5,137,774 and 5,162,147, the disclosures of both of which are herein incorporated by reference, there are described particular coatings of .kappa. phase alumina and/or .alpha. and .kappa. phase alumina including multi-layered coatings. While such products offer significant improvements over single layer .alpha. and/or .kappa. alumina coated cemented carbide inserts, there is a further need for optimization of such products.
It is also known that a .kappa.-alumina coating can be heat-treated to convert the .kappa.-form to the .alpha.-form. Such a heat treatment is disclosed in U.S. Pat. No. 5,071,696, herein incorporated by reference, and produces a fine-grained structure of the .kappa.-alumina. The heat treatment, however, can lead to shrinkage which can lead to cracks in the coating and possible loss by flaking of some or all of the coating layer.
There is thus a need for further improvements in the production of alumina-coated cemented carbide bodies.
In EP Publication 0 603 144 A1 there is disclosed an alumina-coated cemented carbide body in which the alumina coating surface is smoothened by wet blasting.